What is missing from this equation? For one, any data that the “bad thing” might be in the cards. Sure, Tambora did have a VEI 7 (very large) eruption in the recent geologic past. Does this mean that we need to worry about it every time it rumbles? The answer, without any corroborating data, would be a resounding “no.” Unless geologists in Indonesia note a sizable increase in earthquakes, well beyond what might be expected for a smaller eruption (VEI 2-3), significant inflation of the edifice, vast amounts of carbon dioxide or sulfur dioxide emissions or a long string of smaller eruptions that increase in size, well, then likely a “monster” eruption is not in the cards. Tambora might have had a VEI 7 eruption, but it is the exception to the rule in the volcano’s known behavior.

If Lou Merloni hit two home runs in a game yesterday, would he be a safe bet to hit two home runs today? Hint: He hit 14 home runs total in his seven-year career.

This reasoning is kind of like looking for a “hot player” in baseball. Mr. Singles-hitter Second Basemen hit two home runs yesterday after not hitting one in 100 games. Does this mean he is more or less likely to hit today? It is easy to think that he will because he did recently, but there is nothing to support that — and his track record suggests he is more likely to not hit a home run. That is exactly what we examine at volcanoes — the track record over its history, not just what has happened recently.

Now, Tambora is more than a lightweight infielder, but over its known eruptive history over the last 4,000 years, it has had one VEI 7 eruption — and a number of VEI 2 eruptions. Now, we might not have the entire history of Tambora down pat, but we’re likely not missing a VEI 7 event that happened in the last thousand years.

A figure from the Volcanoes of the World — Third Edition (2011) by Seibert and others that shows the relative time intervals between eruptions of different sizes. Note that for VEI 6 or greater, the repose time is typically in the hundreds to thousands of years.

This brings me to my other point: recurrence intervals and repose time — more or less, the average time between eruptions. Now, I know that when it comes to volcanoes, absolute “average time intervals” and a dime will get me a slap in the face. However, there is a fairly well-established correlation between the size of a volcanic eruption and the relative time between eruptions of that size at specific volcanoes (see below). So, a volcano like Etna in Italy has frequent (multiple times a year at times), small (VEI 0-2) eruptions, while someplace like Pinatubo in the Philippines has infrequent (hundreds or thousands of years), large (VEI 5-7) eruptions. The recurrence interval for a VEI 7 eruption is likely somewhere in the thousands of years (see figure below). However, this doesn’t mean a VEI 7 might not occur someplace else in that period, just that if a volcano has a large eruption, the likelihood of another large eruption in a geologically short period of time (for Tambora, less than 200 years) is fairly small.

So, what can we say? Yes, Tambora is showing signs of unrest. The increased seismicity and increased carbon dioxide emissions suggest that an eruption may occur. The PVMBG (the Indonesia Volcanological Agency) has placed Tambora on Alert level 3 (of 4) (in Indonesian) to warn people that an eruption might occur. However, the most probable event is a small explosive eruption or lava flow within the caldera formed during the 1815 eruption, much like what happened in 1880. Think of it like this: You just ran a marathon yesterday. What are the chances you are going to go run another one today? Tambora just ran that marathon in 1815, so maybe it will go out for a little jog, but the chances that we miss a summer in the near future thanks to Tambora are exceedingly low, whether the media likes that news or not.